A navigation device that gives route guidance toward a destination is conventionally known and installation of this navigation device in vehicles is increasingly common. Such a navigation device identifies the present location of a vehicle at any time and stores road shape data on the shape of a road. Accordingly, the navigation device can also determine the curvature, etc. of a curve into which a vehicle is about to enter. Hereupon, the driver of the vehicle judges from the present vehicle speed whether or not the vehicle can safely negotiate the curve, and a system that issues an alarm or controls the deceleration of the vehicle has been proposed in case of necessity.
This proposed system uses a two-dimensional map in which the data of the plane shapes (X and Y) of a road is obtained from a topographical map as the road shape data, and the radius of curvature, etc. of a course is calculated using this two-dimensional map. Meanwhile, the lateral acceleration during turning when the vehicle travels on a curve with a radius of curvature R at a vehicle speed V can be estimated by V.sup.2 /R. Hereupon, the turning safety is evaluated according to this lateral acceleration during turning and the coefficient of friction .mu. between the road surface at that time and the tires of the vehicle. Then, the driver can issue an alarm and control the deceleration, etc. according to the result of evaluation.
Thereupon, to estimate the safety of vehicle movement on such a curve, the present location of the vehicle can accurately be identified and a map database with data, such as curvature (1/ R), a transverse slope (cant: .theta.), and a lengthwise slope at each location on a road, are necessary. In other words, the lateral acceleration during turning ay of a vehicle body is given by EQU ay(.tau.)=(V(.tau.)).sup.2 /R(.tau.)-g.times.sin(.theta.(.tau.))
Where, in consideration of cant .theta. of the road, it is assumed that the vehicle speed in the future (after .tau. seconds) is V (.tau.), the radius of curvature at the location is R (.tau.), and the cant is .theta.(.tau.). At a sharp curve, to facilitate vehicle traveling, a cant of about up to 10% is frequently added, and accurate lateral acceleration can be forecast only when the addition of this cant is considered. Further, because the lengthwise slope of the road also affects the change of vehicle speed and the allocation of vertical tire load on each wheel, the addition of this cant also affects the evaluation of traveling stability.
However, the map database used in the present navigation system includes no cant data. Although the radius of curvature R and the lengthwise slope can be calculated from the location data and altitude data, the calculated R value includes great errors because data spacing is considerably wide. In other words, in the normal map database, location data is only provided for every map point set on a road at intervals of several 10 to several 100 meters, and it was difficult to accurately calculate the shape of a curve.
To forecast curve traveling stability accurately, the map database must be made more accurate. However, immense cost and man-hour would be required to solve this problem. Further, when preparing the map data at fine pitches, there was a problem that the data processing workload increased owing to the large amount of data required to perform the map display and route calculation using the data.
In the Japanese Patent Laid-Open Publication No. Hei 5-238405, a device that stores places requiring attention where the vehicle traveling became unstable due to the vehicle movement obtained when the vehicle actually traveled on the road is proposed. However, this device neither prepares a detailed map database nor accurately forecasts vehicle movement.